1. Field of the Invention
This invention relates to a process for electrodialytically removing at least a portion of the reaction products from an electroless plating bath and more particularly to a process for removing at least a portion of the reaction products and increasing the pH of an electroless copper plating bath.
2. Description of the Prior Art
Electroless metal plating baths that are used commercially generally contain a water soluble salt of the metal to be deposited on the selected substrate and a reducing agent in an aqueous alkaline solution. A complexing or chelating agent is commonly present to prevent precipitation of metal hydroxide. The reducing agent generally is more effective at high pH values, and large amounts of alkali metal hydroxide are consumed in the reduction of the metal salt. When formaldehyde is used as the reducing agent substantially larger amounts of alkali metal hydroxide are consumed because of disproportionation of formaldehyde with hydroxide into formate. The reaction products of the reduction reaction are alkali metal salts, such as sodium formate and sodium sulfate. A build up of these reaction products causes cuprous oxide to form spontaneously within the plating bath leading to general bath decomposition. Also during the plating process substantial amounts of alkali metal hydroxide are consumed since four moles of alkali metal hydroxide are required for each mole of copper deposited. Thus the pH of the plating bath rapidly drops, causing loss of effective plating, i.e., metal deposition, unless the alkali metal hydroxide is replenished. It would be desirable to selectively remove at least a portion of the reaction product anions and also increase the pH of the plating bath whereby only a solution of the metal salt and reducing agent, together with a small amount of the chelating or complexing agent, if needed, is added to the regenerated plating bath to provide a plating bath again suitable for electroless plating.